The primary goal of the proposed subproject is to examine the role of noradrenergic transmitters in the function of the vestibulo-spinal systems. Previous work has shown that there are noradrenergic inputs to portions of the vestibular nuclei, reticular formation, and spinal motor neurons, all of which are presumed to affect vestibulo-spinal reflexes. The first aim seeks to address the issue of whether regions of the vestibular nuclei that contain vestibulo-spinal projecting neurons receive noradrenergic inputs in cats. Immunohistochemical reactions of brainstem sections through the VNC will be performed for two precursor enzymes of norepinephrine (tyrosine hydroxylase and dopamine-beta-hydroxylase) and for the enzyme phenylethanol-N-methyltransferase as a control against epinephrine. The second aim attempts to determine which noradrenergic producing cell groups provide inputs to the vestibular nuclei and whether these cell groups in turn receive reciprocal input from the vestibular nuclei. Neural tracing studies in combination with immunohistochemistry for tyrosine-beta-hydroxylase will be performed to identify the regional projections between the noradrenergic cell groups and the vestibular nuclei. The third aim proposes to determine if the noradrenergic cells that project to the spinal cord, vestibular nuclei and/or reticular formation receive vestibular signals. Electrophysiological recordings from noradrenergic cells will be performed and the cell?s projections identified by antidromic invasion. The response to vestibular and neck stimulation will be examined. The fourth aim is to address the effects of noradrenergic transmitters upon the vestibulo-spinal reflexes. EMG recordings from the triceps muscle will be performed during vestibular stimulation, with application of norepinephrine, as well as noradrenergic agonists and antagonists into the lateral vestibular nucleus.